1. Field of the Invention
The present invention relates to a functional unit positioning device that positions and locks a device or a functional unit, such as a cassette containing a recording medium or a process cartridge, which is detachably attachable to an image forming apparatus body, at a predetermined position in the image forming apparatus body, and an image forming apparatus.
2. Description of the Related Art
It is common to detachably attach various functional units to the main body of image forming apparatuses such as electronic copying machines, printers, and facsimile machines. The image forming apparatus includes a sheet cassette containing recording medium such as transfer sheets. For example, Japanese Utility Model Application Laid-open No. H3-46466 and Japanese Patent Application Laid-open No. 2005-104697 disclose such image forming apparatuses.
FIG. 11 is a plan view of a cassette attachment unit of a conventional image forming apparatus. A locking unit 200 is provided between a paper feeding cassette 211 (hereinafter, “cassette”) and an image forming apparatus body (hereinafter, “apparatus body”) 210 for setting the cassette 211 in a predetermined locked position in the apparatus body 210. FIG. 12 is a plan view of a common locking unit for locking the cassette 211. The locking unit 200 includes a catch mechanism 204 and a locking pawl 205 that engages into the catch mechanism 204. The catch mechanism is a Y-shaped member with a pair of flexible arms 201 and a roller 203 at the end of each of the arms 201. The roller 203 is pivotable around a shaft 202. The locking unit 200 described above needs to have a relatively strong locking force to set the cassette 211 in the predetermined locked position.
A coupling member 251 and a spring 252 are also provided between the apparatus body 210 and the cassette 211. The coupling member 251 conveys a driving force from a driving unit (motor) 250 provided in the apparatus body 210 towards the cassette 211 to turn a base plate 212 provided inside the cassette 211. The spring 252 is attached to a drive shaft 253 of the motor 250 and biases the coupling member 251 towards the cassette 211. The cassette 211 is thus set in the apparatus body 210 in this locked position by the locking unit 200.
The locking unit 200 needs to have enough locking force to securely lock the cassette 211 against an opposing force imposed on the cassette 211 by the coupling member 251 and the spring 252. At the same time, the locking unit 200 needs to have a clamping force for drawing and locking in place the fully loaded cassette 211. Accordingly, the operating force required for locking the cassette in position would include a force required to push and widen the Y-shaped catch mechanism 204 against the clamping force of the locking unit 200 in addition to the force that needs to be exerted against the force exerted on the cassette 211 by the coupling member 251 and the spring 252. The operating force required for locking the fully loaded cassette 211 into place would be a force of about 15 to 20 Newton to counter the opposing force from the spring 252, etc., and about 30 to 50 Newton to counter the force of the catch mechanism 204. When pulling out the cassette 211, however, the opposing force imposed by the spring 252, etc. will not affect the operating force, as the coupling member 251 is in an engaged state.
However, in order for the catch mechanism 204 to exert increased pulling force with the slightest push, a retraction-end cam shape 205a has been given a sharper angle than a locking-end cam shape 205b. Consequently, more force will be required for countering the catch mechanism 204 than for locking. Therefore, the operating force required for pulling out the cassette 211 will be substantially the same as for locking the cassette 211 into place due to the retraction-end cam shape 205a. 
With the advent of the barrier-free trend and universal design, manufactures have been requested to produce machines or tools having improved accessibility for people with disabilities. For example, the United States Government published Accessibility Standard stipulated in Section 508 of the Rehabilitation Act in December, 2000. According to the Accessibility Standard, the maximum operating force for a device or key operation must be 5 pounds weight (22.2 Newton). Thus, the manufactures are required to produce such machines with improved accessibility on the assumption that people with disabilities are employed in any offices.
Accordingly, it is desirable that the operator can insert the cassette into the apparatus body and pull out the cassette from the apparatus body with less operating force. However, the operating force required in the conventional devices is far too high to be deemed acceptable.
The operating force needs to be reduced for not just the cassette but a myriad of other parts such as process cartridge, developer unit, fixing device, toner bottle/toner cartridge, etc. that are detachably attachable to the apparatus body.
However, in the conventional image forming apparatuses, reduced operating force for inserting or removing functional units, etc. translates to inaccurate positioning of the function units. Reduced operating force is desirable for inserting and removing functional units from other devices as well, apart from image forming apparatus.